In many parts of the world, energy customers (including small scale residential customers) are beginning to produce energy locally. This has led to the term “prosumer”—customers who may both consume and produce energy. In many cases, the local energy generation comes from solar photovoltaic (PV) cells that convert sunlight to electricity. Such systems are becoming more and more affordable, and in some cases are very widespread. However, often the solar (or other) energy generation is uncontrollable and unpredictable, and may not occur at the same time as the local energy demand. For example, residential homes often have peak demand in the evening, when the sun is no longer shining. As a result there is now an increasing demand for energy storage, which allows energy generated locally during the day to be stored, so that it can then be used when it is needed later. However, energy storage is expensive and sometimes bulky, and may not be suitable for certain homes.